In optical communication networks, the transmission of multi-channel optical signals over long distances (100 km+) requires the use of optical amplifiers to boost the optical power of the signals, to compensate for signal attenuation experienced during transmission. The gain of each optical amplifier is typically controlled by monitoring the total power of the input signal and output signal of the optical amplifier, and comparing the total output signal power to a predetermined nominal (ideal) total output power. Operation of each optical amplifier is controlled such that the total output power is substantially equal to the nominal total output power. The nominal total output power is the sum of the target optical power for each channel in the multi-channel signal and is determined based on the design requirements of an optical link or network. During the amplification process, the optical amplifiers introduce noise into the multi-channel optical signal and the noise power is measured within the monitored total output power from an amplifier. Therefore, when a multi-channel optical signal is amplified to have a total power equal to the nominal total power, the actual power of each channel is less than the target power because part of the monitored total output power is actually noise and not signal power. The resulting drop in the channel power below the target power is known as channel power depletion.
Common optical amplifiers for the optical transport infrastructure are Erbium doped fibre amplifiers (EDFA) and distributed Raman (Raman) amplifiers. Raman amplifiers are usually used in conjunction with EDFAs in a broad range of applications whose extremes are single-span very long links and ultra-long haul links. The issue of optimally controlling optical network amplifier nodes made of hybrid Raman-EDFA to counteract channel power depletion caused by Raman noise accumulation becomes critical in current reconfigurable transport networks where long and short spans are arbitrarily mixed and the longest all-optical connections must be enabled.
A solution to counteract channel power depletion caused by Raman noise has been proposed in US 20040190123 A1 which accounts for the Raman noise by directly measuring it with a photodiode. However, to do this requires the remote switching off of the traffic channels, complicating the setting up procedure and requiring a coordinated process with remote control. Furthermore, if the Raman gain is required to be changed, for example by changing the pump source power or to adjust the balance between Raman and EDFA gains (manually or through automatic gain controls), then the noise measurement must be redone and this is traffic affecting.